Canned motors are well known in the art and an example of such motors may be seen in U.S. Pat. No. 4,990,068 which is incorporated herein by reference. The term “canned” is derived from the fact that a first metal cylinder or “can” surrounds the rotor (the “rotor can”) while a second metal cylinder fits between the rotor can and the stator (the “stator can”). A small gap, approximately 30/1000 of an inch, is created between the rotor and stator cans which allow cooling fluid to flow between the cans and extract heat from the motor. Additionally, the same fluid typically passes over the rotor shaft bearings on each end of the motor in order to cool and lubricate those bearings.
Canned motors are widely used to power pumps, mixers and the like in the petro-chemical industry. In such an arrangement, the pump or mixer is typically connected directly to the motor body. This eliminates the need for a separate, external seal at the point where the motor shaft engages the pump. Quite often, the fluid being pumped or mixed is also used as the cooling fluid. Because the fluid is often corrosive, sleeve bearings are a typically used in such canned motors since this type of bearing tends to be more resistant to corrosion. However, an inherent disadvantage of sleeve bearings is that they should be rotating at higher motor speeds (e.g. 1800 or 3600 rpm) in order for an effective film of lubricating fluid to form between the sleeves of the bearing. Therefore, at lower motor speeds (e.g. 1200, 900, 600 rpm), sleeve bearings are quickly damaged by failure of effective lubrication. However, the net positive suction head available (NPSHA) in many petro-chemical applications is on the order of 5 to 10 feet of head. Pumps operating at lower motor speeds typically have a lower net positive suction head required (NPSHR) and are more likely to meet the general pump design requirement of the NPSHA being greater than the NPSHR.
An alternative to using canned motor pumps is to use conventional air gap motors (which can effectively operate at lower motor speeds) attached to a separate pump. A common example of this pump arrangement is vertical cantilever styled packed pump, as manufactured by, Lawrence Pumps Inc., Lawrence, Mass. However, this pumping arrangement, as alluded to above, requires the use of a separate seal where the motor shaft engages the pump. Several factors may lead to the failure of these seals which could allow the escape of potentially explosive or toxic materials being pumped.
Furthermore, prior art motor/pump assemblies (whether canned motors or air gap motors) typically have an L3/D4 ratio of 50 or more. The L3/D4 ratio is defined as the overhung shaft length (L) between the axial centerline of the bearing closest to the impeller (inboard bearing) and the axial centerline of the impeller cubed (L3) divided by the shaft diameter (D), defined as the diameter of the smallest cross section within length L exclusive of the impeller mounting surface, raised to the fourth power (D4). However, the larger the L3/D4 ratio, the more shaft deflection which is likely to occur. Such shaft deflection may be generated by any unexpected operating conditions such as pump cavitations, closed suction or discharge valves, or improper operating conditions i.e. improper pump selection. The greater this shaft deflection, the greater the wear on seals and bearings in the system. It would be desirable to have an L3/D4 ratio considerably less than 50.
Another disadvantage of prior art canned motors is their limited axial load thrust capacity and radial load capacity. For example, a 50 or 75 horsepower canned motor with a conventional double acting thrust bearing only has a thrust capacity of approximately 1100 lbs. at 1800 or 3600 rpm. This thrust capacity is considerably reduced if the motor runs at lower speeds. Likewise, a 4.5″ diameter, 3″ long sleeve bearing only has a radial load capacity of 2000 to 3000 lbs. (depending on the fluid used for lubrication). There is a need in the art for canned motors with considerably higher thrust and radial load capacities, both at higher and lower motor speeds.
A further disadvantage of prior art canned motor pumps is that the rotors are axially constrained by the design such that no provision is available for the user to adjust internal axial clearances for wear compensation other than through additional machining of parts, or the addition of shims between mating parts, which modifies the overall length of the assembled components. Both of these methods are costly and time consuming in that they require at least one additional assembly and disassembly of the machine to establish proper operating clearances. Therefore, there also exists a need for a means to adjust the axial position of the rotor, without modification to existing components and without the use of shims, which can be performed as part of a single assembly process.
A further disadvantage of canned motors is that electrical rotor is usually permanently mounted on the rotor shaft. Damage to the rotor shaft sometimes necessitates replacement of the entire rotor include the rotor core, rotor can, and rotor bearings, even though these parts are may not be damaged themselves. This is a costly event. Therefore it would be desirable to be able to replace the rotor shaft without having to replace the rotor core, rotor can, or bearings.